Method and apparatus for scanning sound-film control tracks



Jan. 12, 1943. Q Q D v s 2,307,825

METHOD AND APPARATUS FOR SCANNING SOUND-FTLM CONTROL TRACKS Filed Sept. 24, 1941 TO /6 VAR/A BLE MU TUBE FIG] ' FIG. 3

flllllllllllll||||llllllllllllllmllllllllllimlllillIIIlllllllllllllllllllljlllllll! v I DIRECTION OF TRAVEL Y HJM ATTORNEY-- Patented Jam 12, 1943 METHOD AND APPARATUS FOR SCANNING SOUND-FILM CONTROL TRACKS Charles C. Davis, West Los Angeles, Calii'., asvsignor to Western Electric Company, Incorporated, a corporation of New York Application September 24,1941, Serial No. 412,072

10 Claims.

This invention relates to the scanning of a slow-acting control track recorded in the sprocket hole portion of a photographic film for any desired purpose, an example being the control of the reproduced volume of a sound record occupying a longitudinal portion of the same film.

It is found that for most sound sources the range in volume between loudest and faintest passages is greater than can be successfully recorded on the emulsions now available for the sound negative and the sound positive, the latter being commonly printed on the same film with the corresponding picture. As a result, the reproduced sound is usually compressed in volume range as compared with the original: either the higherlevels are arbitrarily reduced, or the lower levels raised, or both, causing in any event a loss of dramatic effect. It has been proposed to overcome this deficiency by recording, simultaneously with the compressed sound signal, a volume control track registering the variations in gain of the recording electrical circuit. This control track, appearing on the sound positive together with the sound record itself, is relied on to cause automatically the appropriate compensatory variations in reproducing gain to the end that the sound reproduction shall be re-expanded to exhibit the true volume relations of the original sound.

The control track may be located in any 1ongitudinal portion of the film consistent with leaving undisturbed the accepted sound and picture areas, and for reasons of operating convenience the sprocket hole portion has been commonly chosen. This choice, however, introduces the difficulty that the track is interrupted by the sprocket holes. The frequency of these interruptions is 96 per second, corresponding to the standard film speed of 90 feet per minute, and the current in a photocell scanning the sprocket hole control track contains a strong component of this frequency. The sprocket holes, besides.

introducing a 96-cycle component of photocell current, restrict the recordist to the use of a slow-acting control, for the reason that during the passage of the perforation through the scanning light beam no information can be transmitted to the reproducing circuit.

Whatever the type of recording adopted for the control track, whether variable density, variable area, or the so-called direct current track, the 96-cycle component must appear in the reproduction of the control track. It can, of course, be overcome by appropriate electrical means, including usually rectification and filtering. However, all these appurtenances are expensive and cumbersome, and may themselves constitute a hazard to successful sound reproduction.

It is the object of my invention to pr vide an optical arrangement whereby the scanning of the interrupted control track may be rendered continuous, thereby making unnecessary the correcting networks alluded to above.

The apparatus I provide and method I use to effect this object will be readily understood from the detailed description of the accompanying drawing, which shows an embodiment of my-invention with particular reference to a direct current control track:

Fig. l is a schematic view in side elevation of a sound-picture reproducer showing my apparatus in relation to the other parts of the sound reroducer mechanism;

Fig. 2 shows in plan a positive sound-picture film having a direct current control track in the sprocket hole portion with the sound track and picture areas located as usual. In this figure the emulsion coated surface is below the plane of the drawing; and

Fig. 3 is an edge view of the film showing a section thereof taken along the line 3-3 of Fig. 2 and dimensioned to show"the exact relations of the sprocket holes and the elements constituting the embodiment of my invention shown in Fig. 1.

It will be understood that for the sake of greater clarity, the three figures have been drawn to different scales. In all figures the same reference numerals designate corresponding parts.

Referring to Fig. 1, the film l is caused by the usual driving mechanism, not shown, to pass from the feed reel and picture head above through the sound-reproducing apparatus to the take-up reel below. These reels and. the picture projection apparatus are not shown since they form no part of the present invention. Leaving the continuously rotating sprocket 2 of the picture head film I passes, in turn, under guide roller 3, horizontally under plane mirror 4, over guide roller 5, vertically down through the'sound gate 6, under the continuously rotating sprocket l, thence over guide roller 8 to the take-up reel. Rollers 3' and 5' coact with guide rollers 3 and 5, respectively, to provide a slight drag and so insure the film's passing in the desired plane under and parallel to mirror 4.

The remaining structural elements of my invention, coacting with mirror 4 are light source 9, positive lens l0 which renders parallel a por-' tion of the light from source 9, and photoelectric cell II. Photocell II is electrically supplied through conductors I3 and I4 from the battery I5 and polarizing resistance I6, as shown in Fig. 1. The anode and cathode of cell I I are connected, respectively, to the control grid and the cathode of a variable-mu tube, such as the Western Electric 283A or equivalent, as indicated in Fig. 1. This variable-mu tube, not itself a part of the present invention, thus is enabled to con- 'trol the amplification of the sound-reproducing electrical system in accordance with variations in current in cell II which current variations are themselves occasioned by variations in the light returned by mirror 4 through film I to fall on the cathode surface of cell II. The conventional sound-reproducing system comprising lamp I'I, optical system I8, sound gate 6 and photocell I9, together with appropriate amplifiers and loudspeakers, serves for the reproduction of sound track 20 of Fig. 2. The control of the electrical system by a variable-mu tube is well known, being fully explained in U. S. Patent 2,245,652, June 17, 1941, to J. E. Dickert, and need not be described herein. It will be understood that the sound-reproducing system of Fig. 1 is solely i1- lustrative.

In the described embodiment of my invention, the reflecting surface of plane mirror 4 is posi-' tioned approximately parallel to the plane of travel of film I and at a distance therefrom approximately equal to the altitude of an equilateral triangle of sides each equal to half the pitch of the perforations in film I. Light source 9, lens I and photocell II are positioned as shown in Figs. 1 and 3 on the opposite side of the film from mirror 4 and in such relation to each other, to film I and to mirror 4 that the parallel beam of light leaving lens I0 traverses film I and is incident on mirror 4 at an angle of approximately 30 degrees from normal incidence, being reflected from mirror 4 to fall on the photosensitive surface of cell II after again traversing the film. Suitable choice of the size and shape and relative longitudinal location of the film areas efiectively illuminated in the two light passages insures that the control track 2I shall be scanned by the light beam from lens I0, under the condition that no light reaches cell I I which has not been influenced by the control track record. It may be desired to house light source 9, lens I0 and photocell II is an enclosure such as I2 in Fig. 1, leaving as shown an appropriate aperture for exit and re-entry of the scanning light.

Source 9, lens I0, mirror 4 and cell H are so mounted that the optical axis of the direct and reflected beams is in a plane perpendicularto the plane of the film and intersects that plane in the longitudinally central line of the sprocket hole portion. Details of asuitable mounting are not shown, as these are matters of convenient mechanical design. Moreover, it is recognized that those skilled in the art may, if they choose, employ a different geometrical arrangement from that shown in Figs. 1 and 3 without ceasing to use the method of my invention as further defined in the paragraph immediately below.

The method of my invention requires that the film areas effectively illuminated in the two passages of the scanning beam shall be equal rectangles, the centers of which are separated lengthwise of the film a distance greater than the length of the sprocket hole interruption, that is greater than 0.078 inch, but less than the that is, less than 0.109 inch; or, alternatively, a distance selected within these limits, preferably their mean, increased by any convenient multiple of the sprocket hole pitch. It is required that two opposite sides of these rectangules be at right angles to the direction of film travel and approximately equal to the sprocket hole width (0.110 inch), while the sides parallel to the direction of film travel may have any convenient length, the same for both rectangles, preferably not exceeding one-half the perforation pitch, that is, onehalf of 0.187 inch. It is clear that under these conditions the entire width of the sprocket hole control track is scanned and that cell I I receives no light which has escaped the control track.

' Fig. 3 shows, similarly to Fig. 1 but to a larger scale, one form of apparatus by means of which my invention may be applied to effect continuous scanning of .the direct current sprocket hole control track of Fig. 2. Here plane mirror 4 is a rectangle of dimensions 0.0935 inch parallel to the direction of film travel a: 0.110 inch perpendicular thereto, the latter dimension not being shown in Fig. 3. The reflecting surface of mirror 4 is positioned parallel to the plane of the film and perpendicularly distant 0.081 inch therefrom. Since the parallel beam of light from lens I0 is incident on both film I and mirror 4 at an angle of 30 degrees from normal incidence, the dimensions and positioning of mirror 4 serve to constitute this mirror an optical stop: light from lens I0 illuminates the film in the reflected path over an area extending transversely the width of the sprocket holes and longitudinally onehalf the perforation pitch. An equal area is effectively illuminated in the direct path, for the reason that no light traversing film I outside this efiective area is returned by mirror 4 to reach cell II.

In the arrangement of apparatus shown in Fig. 3 the two illuminated areas are longitudinally contiguous with centers separated lengthwise of the film by one-half the perforation pitch. The area ABA'B' of Fig. 2 is continuously illuminated and the light reaching cell I I is wholly under the control of track 2|. It is not, however, essential that the areas illuminated be longitudinally contiguous, nor that their longitudinal extent be onehalf the perforation pitch, nor that their centers be spaced only as shown in Fig. 3. The method of my invention is applied and its object attained by any arrangement which insures that the light reaching the photocell shall have passed twice through the track to be scanned, provided the areas illuminated on the two passages be equal rectangles, with one pair of opposite sides approximately equal to the sprocket hole width and transverse to the direction of film travel, and with centers longitudinally separated on the film by approximately an odd multiple of one-half the perforation pitch. For example, mirror 4 may properly be located three (or five, etc.) times 0.081 inch above the plane of film I; the centers of the areas of light passage will then be separated lengthwise of the film by three (or five, etc.) times one-half the perforation pitch. An optical stop may be introduced in any of the well-known ways to limitthe longitudinal extent of the areas illuminated, if desired, until they become comparable with the conventional sound track scanning line. Moreover, an opaque mask provided with a rectangular slit of appropriate size and shape may be introduced between light source 9 and lens I0, the slit to be imaged by length of film between consecutive sprocket holes, lens I0 on the film, illuminating thereon the and reproduction.

It will be seen thatthe object of my invention is attained by the method and apparatus of the foregoing description. In Fig. 3, control track element CA is eifective to control the current in cell ll while CA lies athwart the direct beam from lens ID, the preceding sprocket hole leaving unobstructed the reflected beam from mirror 4,

and again when CA intercepts the reflected beam, the succeeding sprocket hole replacing CA in the direct beam. Thus, CA undergoes two scannings which slightly overlap (CD=EA'), the effect of sprocket hole interruption is annulled and the control track is scanned as if it were continuous. It will be recalled that a slow-acting control track is contemplated: for such a track the overlap indicated in Fig. 3 is harmless, and the centers of the two scanning areas may be separated by several sprocket holes before the interval between the two scannings becomes a large fraction of the time within which the control track calls for the completion of a change in reproducer gain.

I desire also to point out that my method may be applied toefiect the continuous scanning of a control track over the sprocket holes even where this track is a frequency record, modulated in either amplitude or frequency, provided the scanning areas be reduced in longitudinal extent to be each substantially the conventional sound scanning line. However, in this case special networks are required after the photocell, and the advantage of simplicity is less to be expected.

It will be observed that I have shown inFig. l the structure of my invention in a location above the sound-reproducing system. It may be desired to locate elsewhere the control track scanning system, and this may be done without departing from the spirit of my invention.

While I have described and shown my apparatus and method for scanning a sound film control track interrupted by the sprocket holes, I do not desire to be limited thereto, for it is readily seen that my invention may be adapted to permit effectively continuous scanning of an interrupted longitudinal film track of any type, pro vided the interruptions are periodic and the length of the interrupted portion is not greater than half that of the cycle of interruption.

What is claimed is: 1

1. In a sound film reproducing system, means for scanning a periodically interrupted track occupying a longitudinal portion of a sound film, comprising a light source, an optical system for directing a beam of light from said source to pass through a rectangular area of said track, one dimension of said rectangular area being transverse to said track, a mirror positioned to reflect said beam to return through said track over a rectangular area equal and similarly oriented to the area of first passage through said track, the centers of said rectangular areas being separated longitudinally of said track a distance approximately equal to an odd multiple of half the length of a cycle of interruption, and a light sensitive device receiving said returned beam after its second passage through said track and responsive to variation of the light flux in said returned beam.

2. In a sound film reproducing system, means for scanning a periodically interrupted track occupying a longitudinal portion of a sound film. comprising a light source, an optical system for directing a beam of light to pass through an area of said interrupted track, a mirror positioned to reflect said beam to return through an equal area of said track, the centers of said areas being longitudinally separated by approximately half the length of a-cycle of interruption, and a light sensitive device receiving said returned beam and responsive to variation'of the light flux therein.

3. In a sound film reproducing system, means as in claim 2 for scanning a periodically interrupted track occupying a longitudinal portion of a sound film, wherein each area of light passage through said interrupted track is a rectangle of width approximately equal to that of said track and of length not greater than one-half the length along said track occupied by a complete cycle of interruption.

4. In a sound film reproducing system, means as in claim 2 for scanning a periodically interrupted track occupying a longitudinal portion of a sound film, wherein each area of light passage through said interrupted track is a rectangle of width approximately equal to that of said track and of length not greater than one-half the length along said track occupied by a complete cycle of interruption, and wherein the axes of the direct and reflected light beams are each incident on said track at an angle approximately 30 degrees from normal incidence.

. 5. In a sound film reproducing system, means for scanning a control track in the sprocket hole portion of a sound film, comprising a light source, an optical system for directing a beam of light from said source to pass through a rectangular area of said track, one dimension of said rectangular area being transverse to said track, a mirror positioned to reflect said beam to return through said track over a rectangular area equal and similarly oriented to the area of first passage through said track, the centers of said rectangular areas being separated longitudinally of said track a distance approximately equal to an odd multiple of half the distance between centers of consecutive sprocket holes, and a light sensitive device receiving said returned beam after its second passage through said track and responsive to variation of the light flux in said returned beam.

6. In a sound film reproducing system, means for scanning a control track in the sprocket hole portion of a sound film comprising a light source, an optical system for directing a beam of light to pass through an area of said sprocket hole portion of the film, a mirror positioned to reflect said beam to return through said portion of the film over an area equal to that of first passage, the centers of said areas being longitudinally separated by approximately half the sprocket hole pitch, and a light sensitive device receiving said returned beam and responsive to variation of the light flux therein.

'7. In a sound film reproducing system, means as in claim 6 for scanning a control track in the sprocket hole portion of a sound fihn, wherein each area of light passage through said control track is a rectangle to that of the sprocket holes and of length not greater than one-half the sprocket hole pitch.

of width approximately equal.

8. In a sound film reproducing system, means as in claim 6 for scanning a control track in the sprocket hole portion of a sound film, wherein each area of light passage through said control track is a rectangle of width approximately equal to that of the sprocket holes and of length not greater than one-half the sprocket hole pitch, and wherein the axes of the direct and reflected light beams are each incident onsaid volume control track at an angle approximately 30 degrees from normal incidence.

9. In a sound film reproducing system, the method of continuously scanning a periodically interrupted track occupying a longitudinal porequal to an odd multiple of half the length of a cycle of interruption, and receiving said returned beam after its second passage through said track on a photosensitive device responsive to variation of the light flux in said returned beam.

10. In a sound film reproducing system, the method of continuously scanning a control track in the sprocket hole portion of a sound film which comprises directing a beam of light through said control track, illuminating thereby on said control track a rectangular area having one dimension transverse to said track and approximately equal to the width thereof and extending lengthwise of said track a distance no greater than onehalf the sprocket hole pitch,-reflecting said transmitted beam to return through an equal and similar area of said track, the centers of said areas being longitudinally separated a distance approximately equal to an odd multiple of half the sprocket hole pitch, and receiving said returned beam after its second passage through said track on a photosensitive device responsive to variation of the light flux in said returned beam.

CHARLES C. DAVIS. 

